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Subjectquestion regarding strange kernel stats under high (network) load
Date
Hi,

for a network monitoring project that I'm working on (see my sig if you're
interested) I have to compare system load under high throughput. Directly
comparing my implementation to tcpdump (LSF) seems fine, our user% and
kernel% are lower. However, when I added an idle system I noticed some
strange behaviour that remains on re-executing the tests:

the kernel% is actually higher when nothing is running than when my code is
running! Overall processing is lower (idle% is higher on the idle system),
but that is thanks to the high %si (soft ints) that my code generates.

In order to better explain my problem here is the output of top (the new one,
with %si, %hi and %wa) for a 600Mbit throughput, the first line is for the
idle system, the second for FFPF (my system) and the third for Linux Socket
Filters:

Mbit iperf in iperf out capt recv kern drop % drop tcpdump % iperf % user %
kernel % idle %
600 608 608 33 6 30 40
600 608 608 514084 517314 33 0.63 17 20 13 15 25
600 609 608 421215 517597 96382 31.4 76 33 17 49.5 0

the usage statistics are user%, kernel% and idle% (I haven't saved the si%
results). What's odd is that although idle is higher for the idle system,
kernel% is actually lower. I can understand that my code generates extra soft
interrupts, but not that it has an effect on the `other' kernel load. Perhaps
I don't understand the values good enough, in which case I would greatly
appreciate your help. However, it could also signal a bug in the statistics
reporting (although I don't hope that's the case, these results are great for
my paper :). FYI, I used the command

top -b -n 5 -d 3 | grep "Cpu\|tcpdump\|iperf"

to generate these results.

thanks for your help,

Willem
--
Willem de Bruijn
wdebruij_at_dds.nl
http://www.wdebruij.dds.nl/

current project :
Fairly Fast Packet Filter (FFPF)
http://ffpf.sourceforge.net/

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